How to position an object in the middle of of two parts of another object
Select the top and bottom edge of the area you want to place the circle in the middle of. Press Shift+S and select &quot;cursor to selected&quot;. As long as you have your Transform Pivot Point still set to &quot;median&quot;, this will place the cursor in the center between the two selected edges.

Now when you add the circle, it will be placed at the point of the cursor (the middle).
From there, just rotate and scale the circle to your desire.